Foursquare is probably the most popular location-based social application today, and developer Eric Butler has created FoursquareX so Mac users can keep up with their foursquare friend's movements.
Currently available for Snow Leopard only, FoursquareX uses the CoreLocation API to work out your position based on wifi networks around you, so your Mac doesn't need to grow a GPS function! The app itself is pretty simple from there - you can check into locations, and get Growl updates as your friends check in or leave shouts. You can also connect with Facebook and Twitter to post your location and thoughts.
The mobile apps have more features at the moment, but that may reflect the fact that Foursquare is not meant to be a stationary thing! The main problems with FoursquareX stem from Foursquare itself - that outside a few US cities its not popular enough yet to be useful, and that you can check into any location, regardless of your actual position, which seems a bit silly.
Even in early development, FoursquareX is stable and easy to use. If you're addicted to it now you can check into Foursquare even if you lose your phone.